The rotary anode type X-ray tube has a structure that an anode target for radiating X-rays is rotatably supported by a rotating mechanism, and an electron beam is emitted to the anode target rotating at a high speed to irradiate the X-rays from the anode target. The rotating mechanism for supporting the anode target is comprised of a rotary body, a stationary body and the like, and bearings are disposed between the rotary body and the stationary body. For a bearing section, there is used a rolling-element bearing such as a ball bearing or a dynamic pressure type sliding bearing which has helical grooves formed on a bearing surface and supplies a liquid metal lubricant such as gallium (Ga) or gallium (Ga)-indium (In)-tin (Sn) alloy to the helical grooves and the like.
A conventional rotary anode type X-ray tube will be described with reference to FIG. 21. The rotary anode type X-ray tube shown in FIG. 21 has a dynamic pressure type sliding bearing in a bearing section. A rotary anode type X-ray tube 1 shown in FIG. 21 is housed in a housing vessel 2. The rotary anode type X-ray tube 1 has a cathode 4 and an anode target 5 mutually opposed in a vacuum vessel 3. The anode target 5 is connected to a rotary shaft 7 or the like of a rotating mechanism 6 and rotatably supported by the rotating mechanism 6. The rotating mechanism 6 has a rotary body 8 connected to the rotary shaft 7 and a stationary body 9 fitted to the rotary body 8.
The bottom end opening of the rotary body 8 is sealed with a thrust ring 10, and the stationary body 9 is extended to the outside through the thrust ring 10. And, a hole 11 forming a cooling passage through which a cooling medium flows is formed in the stationary body 9 along the tube axis. The bottom end of the stationary body 9 is connected air-tight to fix to a sealing ring 12 for sealing one end of the vacuum vessel 3. And, a stator 13 is disposed outside of the vacuum vessel 3.
When the rotary anode type X-ray tube 1 operates, the rotary body 8 and the anode target 5 are rotated at a high speed by a rotating magnetic field generated by the stator 13. Under this condition, an electron beam e generated by the cathode 4 is accelerated by a high voltage between the anode and the cathode and impinged on the anode target 5 to emit X-rays. The X-rays are guided outside as indicated by arrow Y through an output window W1 disposed on the vacuum vessel 3 and an output window W2 disposed on the housing vessel 2.
The anode structure of the above-described rotary anode type X-ray tube 1 will be described with reference to FIG. 22. The rotary body 8 has a three-layered structure comprising an intermediate cylinder 8a connected to the rotary shaft 7, an inner cylinder 8b connected to the inside of the intermediate cylinder 8a, and an outer cylinder 8c connected to the exterior of the intermediate cylinder 8a. A blackened film 14 for radiating heat is formed on the exterior surface of the outer cylinder 8c. The bottom end opening of the inner cylinder 8b is sealed with the thrust ring 10.
The stationary body 9 is fitted into the inner cylinder 8b of the rotary body 8. The bottom end of the stationary body 9 passes through the thrust ring 10 to extend below it. Dynamic pressure type sliding bearings are disposed in the fitted surface of the inner cylinder 8b and the stationary body 9. For example, radial dynamic pressure type sliding bearings 15a, 15b are disposed on two remote portions in a tube axial direction on the external surface of the stationary body 9. And, dynamic pressure type sliding bearings 16a, 16b are disposed in a thrust direction on the top end surface of the stationary body 9 and the bottom stepped surface of the stationary body 9.
Herringbone pattern helical grooves 17 as shown in FIG. 23 are formed in pair on the radial dynamic pressure type sliding bearings 15a, 15b. Herringbone pattern helical grooves 18 are formed on the dynamic pressure type sliding bearings 16a, 16b in the thrust direction as shown in FIG. 24. A liquid metal lubricant of gallium or gallium alloy is supplied to the helical grooves 17, 18 and the gap of the fitted portion between the inner cylinder 8b and the stationary body 9. A non-bearing area 19, which is provided in the area interposed between the two bearings 15a, 15b disposed on the external surface of the stationary body 9, has a larger gap between the inner cylinder 8b and the stationary body 9 than those formed between the bearings 15a, 15b and the inner cylinder 8b and does not operate as a bearing. The gap of the non-bearing area 19 serves as, for example, a liquid metal lubricant storage section.
The gap between the inner cylinder 8b and the stationary body 9 is determined to have a bearing size so that the rotary body 8 can rotate stably. A specific size of the gap for the bearing section is variable depending on a rotating speed of the rotating part, a shape of the shaft, or the like. For example, the dynamic pressure type sliding bearings 15a, 15b have the gap between the inner surface of the inner cylinder 8b and the outer surface of the stationary body 9 determined to be about 1/1000 or less of the diameter of the stationary body 9 at the bearing sections as shown in FIG. 22.
When the rotary anode type X-ray tube 1 operates, the temperature of the anode target 5 increases by the irradiation of the electron beam. To release the heat of the anode target 5 to outside of the X-ray tube 1, the rotary anode type X-ray tube 1 using the dynamic pressure type sliding bearing employs the following method (see Japanese Patent Laid-Open Publications No. HEI 2-244545, No. HEI 5-144395, No. HEI 6-76772, No. HEI 7-226177 and No. HEI 9-171789, U.S. Pat. No. 5,838,763, etc.). For example, the heat of the anode target 5 is conducted to the rotary body 8 connected to the anode target 5. The heat is then conducted from the rotary body 8 to the stationary body 9 through the liquid metal lubricant in the bearing portions. And, the heat is radiated from the stationary body 9 to the outside of the X-ray tube 1. A passage for the cooling medium is formed in the stationary body 9.
According to the above-described method of releasing the heat from the conventional rotary anode type X-ray tube, an increase in temperature of the bearing section becomes great. For example, in the anode structure shown in FIG. 22, the heat of the anode target 5 is conducted from the rotary shaft 7 to the intermediate cylinder 8a and from the intermediate cylinder 8a to the stationary body 9 through the inner cylinder 8b and the bearing 15b. Thus, the heat of the anode target 5 is directly conducted to the bearing section, so that the increase in temperature of the bearing section becomes great.
Besides, as described in Japanese Patent Laid-Open Publication No. HEI 2-244545 and U.S. Pat. No. 5,838,763, when the anode target and the rotary body are mutually contacted to have a large contact area, quantity of heat conducted from the anode target to the rotary body increases, and the temperature of the bearing section becomes higher. As a result, there is a problem that the bearing-forming material and the liquid metal lubricant react to each other to make the bearing surface rough or the bearing gap size is varied by the reaction product. Such a problem has an adverse effect on the bearing operation, and the bearing operation cannot be maintained stably.
When the rotating anode type X-ray tube starts to operate, the rotary section of the rotating mechanism rotates, shearing energy is applied to the liquid metal lubricant and changes to heat, and the bearing section generates heat. The heat from the anode target is also applied to the bearing section as described above. Thus, Japanese Patent Laid-Open Publication No. HEI 7-226177 and Japanese Patent Laid-Open Publication No. HEI 9-171789 disclose a structure that a connecting portion having a heat resistant structure such as a cylinder is disposed between the anode target and the rotary body to decrease heat to be conducted to the rotary body.
To dispose the connecting portion, the anode target is connected to the connecting portion, and the connecting portion is connected to the rotary body of the rotating mechanism. For the coupled portion between the anode target and the connecting portion and the coupled portion between the rotary body and the connecting portion, either of them is integrally formed, and the other is mechanically contacted and fixed with a screw part. Such a coupling structure has high heat resistance, and an adequate heat transfer effect cannot be obtained. Magnitude of heat resistance is variable depending on the surface roughness of the contact portion, and a heat resistance value is variable depending on processing accuracy. When the heat resistance value is variable, for example, a temperature difference is caused in the rotary body within one tube or a temperature difference is caused in the rotary body depending on a tube. In such a case, it is necessary to design considering variations in temperature, and enhancement of the heat transfer effect is disturbed.
Besides, according to the conventional rotary anode type X-ray tube, to transfer the heat of the anode target from the rotary body to the stationary body through the bearing surface, the heat is finally transferred to the cooling medium flowing through the cooling passage formed in the stationary body. Quantity of heat transferred to the cooling medium becomes larger as the effective contact area between the stationary body and the cooling medium becomes larger. The effective contact area depends on a portion where heat is transferred effectively (e.g., a part of the cooling passage having a high surface temperature) in the surface area of the cooling passage. But, heat of the anode target is transferred to the cooling medium through a small region having a short distance among the heat routes from the rotary body to the cooling passage in the stationary body. Then, the effective contact area cannot be made adequately large, resulting in degradation of the operation characteristics of the bearing section.
This invention provides a rotary anode type X-ray tube which basically makes good conduction of heat of the anode target through a rotating mechanism and the like. Specifically, it is to provide a rotary anode type X-ray tube which suppresses the dynamic pressure type sliding bearing section from having a temperature increase and can maintain a stable bearing operation. And, it provides a rotary anode type X-ray tube which suppresses variations in heat transferred from the anode target to the rotary body of the rotating mechanism. Besides, it provides a rotary anode type X-ray tube having an improved heat transfer characteristic from the rotary body of the rotating mechanism to the stationary body.